Processes for preparing oxide powders and superconducting oxides

ABSTRACT

Fine and homogeneous oxide particles for superconductors which can be sintered at a low temperature are prepared in a liquid phase by the sol-gel method using alkoxides as starting materials. By forming a buffer layer between a substrate and a superconducting film, good-quality and oriented superconducting film can be fabricated. Highly c-axis-oriented superconducting film and bulk products can be prepared from particular starting compositions in Ln-Ae-Cu-O and Bi-Ae-Cu-O systems. The oriented film can be produced by painting a paste of such starting compositions on a substrate followed by sintering, and the bulk form can be produced by pressing the pre-sintered powder of such starting compositions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing oxide powderswhich are used as starting materials for producing high temperaturesuperconducting ceramics and to a process for fabricating thesuperconducting oxide ceramics.

The present invention also relates to processes for producing c-axisoriented superconducting film and bulk materials.

2. Background of the Related Art

Recently, superconducting oxide ceramics which show superconductivity ator above the liquid nitrogen temperature have been found. For example,YBa₂ Cu₃ O_(7-x) and LnBa₂ Cu₃ O_(7-x) (Ln: a lanthanoid) have beenreported to be superconductors having a superconducting transitiontemperature (T_(c)) at about 90K. These high-T_(c) superconductingoxides are favorable for wide commercial application, because the priceof the liquid nitrogen used as the cooling medium is about 1/10 of thatof liquid helium. These superconductors are fabricated from oxidepowders as starting materials. Since the properties of thesuperconductors themselves are strongly influenced by the startingmaterials, not only is the fabrication process for the superconductorsbeing widely investigated, but also the method for producing thestarting oxide powders.

A solid phase reaction method and a coprecipitation method are knowntechniques for producing oxide powders for superconductors. Thefabrication process of the solid phase reaction method is as follows; amixture of oxides, such as CuO, Y₂ O₃ (or Ln₂ O₃) and BaCO₃, in acertain ratio is sintered at a temperature of 950°-1000° C. to make YBa₂Cu₃ O_(7-x) or YLn₂ Cu₃ O_(7-x) in a solid phase reaction at a hightemperature, and then the sintered product is slowly cooled to roomtemperature and is ground into powder. The fabrication process of thecoprecipitation method is as follows; oxalic acid is added to an aqueoussolution of Cu(NO₃)₂, Ba(NO₃)₃ and Y(NO₃)₃ or Ln(NO₃)₃ to precipitatemetallic ions as oxalates, and then the resultant precipitants are driedand calcinated at a temperature of 900°-1000° C. to decompose theoxalates into oxides, and the products are slowly cooled and ground intopowder.

In the solid phase reaction method, a one hundred percent reaction ofthe components cannot be accomplished and, hence, unreacted oxides areoften present in the product. On the other hand, while in thecoprecipitation method more homogeneous products can be obtained than inthe solid phase reaction method, the starting composition and thecomposition of the products are different, which is caused by the highersolubility of barium oxalate than the solubility of the othercomponents.

From the point of view of crystalline phase change, in both of thesetechniques, a non-superconducting phase (tetragonal phase) appears uponsintering at 900°-1000 ° C. during the first stage. Then, the tetragonalphase is transformed into a superconducting phase (orthorhombic phase)by absorption of oxygen during the subsequent slow cooling process in anoxygen-rich atmosphere. The reason why a sintering temperature higherthan 900° C. is necessary is that the large particle size of thestarting powders made by these techniques results in low activity in thereaction.

Moreover, after grinding of the calcined products, the particle size ofthe powder is on the order of a few micrometers and the sizedistribution is not as sharp. Therefore, to make a dense product fromthe powder made by these techniques, re-sintering at a temperaturehigher than 1000° C. is required. Since such a high temperaturesintering often generates other crystal phases (not the YBa₂ Cu₃ O_(7-x)or LnBa₂ Cu₃ O_(7-x) phase), however, sometimes the T_(c) of the samplewas lower than the liquid nitrogen temperature.

Meanwhile, for forming a superconducting film, various techniques havebeen investigated including a method using a reaction in the gas phase,such as the plasma spray coating technique, a vacuum evaporationtechnique, a sputtering technique, and coating methods, such as thescreen printing technique and the spin coating technique. Thesetechniques include a heating process after the formation of the film ona substrate, and have some problems. In the vacuum evaporation and thesputtering techniques, the composition of the film is often differentfrom the starting material composition. Furthermore, in the sputteringprocess, crystal structure and orientation of the resultant film arestrongly influenced by the substrate temperature. The screen printingtechnique also has the problems that a dense film cannot be obtainedeasily because of the large particle size of the starting powder, andthat the critical current density of the film made by this technique islower than that of a film formed by sputtering. Another technique forproducing super-conducting film has been proposed; organometalliccompounds, such as yttrium stearate, barium naphthenate and coppernaphthenate, are dissolved into a suitable solvent and are coated on asubstrate and heated to decompose into an oxide film. This technique,however, has the problem that carbon remains in the film because of thestrong reducing atmosphere in the sintering process.

When silicon or silica glass are used as substrates for superconductingoxide films, the superconducting phase cannot be obtained because of thereaction between the copper in the oxide film and the substrates duringthe sintering process. Accordingly, substrates having low reactivityduring sintering, such as yttrium-stabilized zirconia (YSZ) or the like,are used as a substrate for film forming. Such a type of substrate,however, has the following problems: (1) since the substrate is aninsulator, for a superconducting device involving circuit application,another substrate which is semiconducting is required; and (2) since itis difficult to make variously shaped substrates, such as fiber or tape,from ceramics, the substrate shape is limited to that of a plate.

For practical application of the superconducting material, a highcritical current density and a high critical magnetic field should besecured. It is known that if the superconducting crystal is c-axisoriented and a current is flowed along a surface perpendicular to thec-axis, critical current density is improved. Therefore, it is desirablethat the superconducting crystal have a particular orientation.Although, when the YBa₂ Cu₃ O_(x) powder is pressed into a pellet shape,c-axis orientation has been observed, no method has been reported torealize c-axis orientation effectively.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forproducing a homogeneous superconducting oxide powder which has a smallparticle size and can be sintered at a low temperature for solving theabove-mentioned problems.

It is another object of the present invention to provide a process formaking a high T_(c) superconducting oxide by sintering at a lowtemperature.

It is a further object of the present invention to provide a layeredsuperconducting structure formed on a substrate which has goodsuperconducting properties without any particular limitation regardingsubstrate material.

It is a still further object of the present invention to provide aprocess for forming a superconducting oxide film or bulk product havinga high degree of c-axis orientation.

In a first aspect of the present invention, a process for preparing anoxide powder, comprises the steps of:

separately dissolving corresponding raw material alkoxides of aplurality of elements to constitute a super-conducting oxide in asolvent;

mixing a plurality of the resulting solutions together;

subjecting the resulting mixed solution to hydrolysis to give a sol; and

evaporating the solvent in the sol.

In a second aspect of the present invention, a process for preparing anoxide powder, comprises the steps of:

separately dissolving corresponding raw material alkoxides of aplurality of elements to constitute a super-conducting oxide in asolvent;

mixing the resulting solutions, subjecting the resulting mixed solutionto hydrolysis to give a sol, and evaporating the solvent in the sol toprepare a condensed solution;

forming the concentrated solution into a continuous body; and

sintering the continuous body.

Here, the plurality of elements may be yttrium or at least onelanthanoid, at least one alkali earth metal and copper.

A dilute solution of an alkoxide of yttrium or a lanthanoid prepared byan exchange reaction of its chloride with a sodium alkoxide may be usedas the raw material of yttrium or lanthanoid. The plurality of elementsmay be at least one alkali earth metal, bismuth and copper.

At least one operation among dissolution and hydrolysis may be conductedwhile refluxing the solution or suspension. The solution or suspensionafter hydrolysis may be kept at a given temperature to effect agingthereof.

A powder of the oxide may be mixed with an oxide(s) or carbonate(s) ofindividual element(s) which constitute a superconducting oxide, followedby forming and baking.

The continuous body may have bulk form. The continuous body may be afilm.

The film may be formed on a buffer layer provided on a substrate. Thebuffer layer may be a layer comprising zirconia as the main component.

The oxide may have a composition for which Ln/(Ln+Ae+Cu)<16.6 mole %,Cu/(Ln+Ae+Cu)>50 mole %, and Ae=the balance, wherein Ln is yttrium or atleast one lanthanoid, and Ae is at least one alkali earth metal.Further, a powder of the oxide may be pressed to orient the c-axes ofits crystals in parallel with the pressing direction.

The oxide may have a composition for which Bi/(Bi+Ae+Cu)=5 to 40 mole %,Ae/(Bi+Ae+Cu)=15 to 70 mole %, and Cu/(Bi+Ae+Cu)=24 to 64 mole %,wherein Ae is at least one alkali earth metal. Further, a powder of theoxide may be pressed to orient the c-axes of its crystals in parallelwith the pressing direction. The powder may have a composition for whichBi/(Bi+Ae+Cu)=5 to 40 mole %, Ae/(Bi+Ae+Cu)=15 to 40 mole %, andCu/(Bi+Ae+Cu)=24 to 64 mole %.

In a third aspect of the present invention, a layered superconductingoxide structure is characterized by comprising:

a substrate;

a buffer layer formed on the substrate and having a low reactivity withthe material of the substrate; and

a superconducting oxide film formed on the buffer layer.

Here, the buffer layer may be an oxide film consisting of zirconia orcomprising zirconia as the main component.

In a fourth aspect of the present invention, a process for preparing asuperconducting oxide, comprises the steps of:

sintering an oxide powder to develop a superconducting phase;

grinding the resulting sinter into a fine powder; and

pressing the fine powder to develop c-axis orientation in a directionparallel with the pressing direction.

Here, the oxide powder may have a composition for whichLn/(Ln+Ae+Cu)<16.6 mole %, Cu/(Ln+Ae+Cu)>50 mole %, and Ae=the balance,wherein Ln is yttrium or at least one lanthanoid, and Ae is at least onealkali earth metal.

The oxide powder may have a composition for which Bi/(Bi +Ae+Cu)=5 to 40mole %, Ae/(Bi+Ae+Cu)=15 to 70 mole %, and Cu/(Bi+Ae+Cu)=24 to 64 mole%, wherein Ae is at least one alkali earth metal.

The oxide powder may have a composition for which Bi/(Bi +Ae+Cu)=5 to 40mole %, Ae/(Bi+Ae+Cu)=15 to 40 mole %, and Cu/(Bi+Ae+Cu)=24 to 64 mole%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction pattern of a powder prepared by thesol-gel method in accordance with the present invention;

FIG. 2 shows change of porosity with treatment temperature for samplesprepared according to the present invention;

FIG. 3 shows the relationship between specific surface area andtreatment temperature for a powder mixture made by the sol-gel method inaccordance with the present invention and a commercially availablepowder mixture;

FIGS. 4A, 4B and 4C show X-ray diffraction patterns for samples sinteredat 750, 800° and 950° C.;

FIG. 5 shows an X-ray diffraction pattern of a zirconia film formed onan Si substrate according to the present invention;

FIG. 6 shows an X-ray diffraction pattern of a sintered Y-Ba-Cu-O oxidefilm formed on a substrate;

FIG. 7 shows an electrical resistance versus temperature curve for afilm made in accordance with the present invention;

FIG. 8 shows an X-ray diffraction pattern for a c-axis-orientedY-Ba-Cu-O film formed on a YSZ substrate;

FIG. 9 shows an electrical resistance versus temperature curve for thesample which is shown in FIG. 8;

FIG. 10 shows change in composition after heat treatments;

FIG. 11 shows compositions of the starting Y-Ba-Cu-O compounds made bythe sol-gel method in accordance with the present invention;

FIG. 12 shows an X-ray diffraction pattern for a partially orientedsample;

FIG. 13 shows compositions of the starting Y-Ba-Cu-O mixtures ofcommercially available powder reagents;

FIG. 14 shows an X-ray diffraction pattern for a c-axis-orientedNd-Ba-Cu-O film sample;

FIG. 15 shows an X-ray diffraction pattern for a c-axis-orientedY-Nd-Ba-Cu-O film sample;

FIG. 16 shows starting composition in a Bi-Sr-Ca-Cu-O system;

FIG. 17 shows an X-ray diffraction pattern for an oriented Bi₂ (Sr, Ca)₃Cu₂ O_(x) film sample in accordance with the present invention;

FIG. 18 shows an electrical resistance versus temperature curve for ac-axis-oriented Bi-Sr-Ca-Cu-O sample;

FIG. 19 shows an X-ray diffraction pattern for a highly oriented Bi₂(Sr, Ca)₃ Cu₂ O_(x) film sample;

FIGS. 20A and 20B show X-ray diffraction patterns for the powder samplesmade from the orientation composition and the 1-2-3 composition,respectively;

FIG. 21 is a schematic view illustrating c-axis orienting by pressing;

FIG. 22 shows the relationship between the degree of c-axis orientation(peak ratio of (006)/(110)(103)) and the applied pressure;

FIG. 23 shows the compositions from which high orientation is easilyobtained by pressing;

FIG. 24 shows the relationship between the orientation (peak ratio of(006)/(110)(103)) and the applied pressure;

FIG. 25 shows an X-ray diffraction pattern for a powder sample preparedfrom alkoxides;

FIG. 26 shows an X-ray diffraction pattern for a c-axis-orientedBiSrCaCu₃ O_(x) powder sample;

FIGS. 27A, 27B, and 27C show X-ray diffraction patterns for samples madeby pressing at different pressures; and

FIG. 28 shows relationships between the orientation (presented by peakratio (008)/2Θ=27.5°) and the applied pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sol-gel method using alkoxides as starting materials in accordancewith the present invention will be presented below.

In the present invention, for preparing superconducting powder,alkoxides are dissolved into solvents and mixed. Then, they arehydrolyzed and dried.

According to the process of the present invention, since raw materialsare dissolved into the solvent and hydrolyzed into fine, complex oxideparticles in a liquid phase, the resultant particles are very small insize and homogeneous. Thus, these powders made by the process of theinvention have higher activity for sintering than those made byconventional processes and are obtained by reaction at lowertemperature.

Alkoxides M(OR)_(n) (where M is metal, OR is an alkoxyl group and n isthe valence state of metal M) are hydrolyzed by water into metallicoxides (or their hydrates) and an alcohol, as shown in the followingreaction equations

    M(OR).sub.n +nH.sub.2 O→M(OH).sub.n +nROH

    mM(OH).sub.n →mMO.sub.n/2 +mn/2 H.sub.2 O.

If two or three alkoxides are hydrolyzed simultaneously, fine, double ortriple complex oxide particles can be formed at a temperature below theboiling point of the solvent.

Generally, the powders prepared by hydrolysis of alkoxides have a smallsize ranging from 0.001 to 0.01 μm. This means that the powder has alarge surface energy and shows a good packing ability. Therefore, adense product can be obtained by sintering at relatively lowtemperature.

The processes of the present invention will be presented below in detailincluding: (1) preparation of a solution, (2) hydrolysis, (3) aging and(4) drying processes.

(1) Preparation of a Solution

Since, in general, alkoxides of copper, barium, yttrium and lanthanoidsare all solids at room temperature, solubility of the alkoxides inorganic solvents is one of the key points in selection. If thesolubility of the alkoxide is very low, an enormous amount of solvent isnecessary to hydrolyze and polymerize uniformly, which is far fromeconomical. The solubility of alkoxide powders in 1 liter of varioussolvents is determined by experiment (Table 1).

                  TABLE 1                                                         ______________________________________                                        Solubility of Alkoxide Powders                                                                       Solubility (g) in 1 l                                  Alkoxide     Solvent   of Solvent (20° C.)                             ______________________________________                                        Cu(OCH.sub.3).sub.2                                                                        CH.sub.3 OH                                                                             10                                                     Cu(OC.sub.2 H.sub.5).sub.2                                                                 C.sub.2 H.sub.5 OH                                                                      12                                                     Cu(i-OC.sub.3 H.sub.7).sub.2                                                               i-C.sub.3 H.sub.7 OH                                                                    15                                                     Ba(OCH.sub.3).sub.2                                                                        CH.sub.3 OH                                                                             0.04                                                   Ba(OC.sub.2 H.sub.5).sub.2                                                                 C.sub.2 H.sub.5 OH                                                                      0.07                                                   Ba(i-OC.sub.3 H.sub.7).sub.2                                                               i-C.sub.3 H.sub.7 OH                                                                    0.09                                                   Y(OCH.sub.3).sub.3                                                                         CH.sub.3 OH                                                                             0.01                                                   Y(OC.sub.2 H.sub.5).sub.3                                                                  C.sub.2 H.sub.5 OH                                                                      0.01                                                   Y(i-OC.sub.3 H.sub.7).sub.3                                                                i-C.sub.3 H.sub.7 OH                                                                    0.02                                                   ______________________________________                                    

As shown in Table 1, the solubility of barium and yttrium alkoxides in 1liter of solvent are on the order of several ten mg, which is onehundredth of that of copper alkoxides. The solubility of lanthanoids areon the same order as those of yttrium alkoxides. These results indicatethat it is impractical to use dried powder alkoxides of barium, yttriumor lanthanoids as starting materials because of their low solubilities.

Nevertheless, it was found in an experiment that alkoxides can bedissolved in a solvent just after synthesis. For example, bariumalkoxide, which can be obtained by direct reaction of metallic bariumwith an alcohol using a reflux technique as shown in the followingreaction equation, can be dissolved (several tens of grams) in 1 literof an alcohol solvent before cooling:

    Ba+2ROH→Ba(OR).sub.2 +H.sub.2.

Yttrium or lanthanoid alkoxides which can be synthesized by an exchangereaction of yttrium or lanthanoid chlorides with a sodium alkoxide usingthe reflux technique in an alcohol solvent (given in the followingreaction equation), can also be dissolved (several grams) in 1 liter ofthe alcohol solvent before cooling:

    YCl.sub.3 +3NaOR→Y(OR).sub.3 +3NaCl

    LnCl.sub.3 +3NaOR→Ln(OR).sub.3 +3NaCl.

Although the basis for these phenomena is not clear, if, aftersynthesizing, barium, yttrium or lanthanoid alkoxides are usedimmediately, completely dissolved alkoxides are available for the nextreaction. It should be noted that in order to avoid cooling, thealkoxide solutions must be treated immediately after synthesizing. Analcoholic solution containing two equivalents of barium alkoxide ismixed with a solution containing one equivalent of yttrium alkoxide withno cooling. And, then, three equivalents of copper alkoxide solution areadded with vigorous stirring. Thus, a starting solution is obtained. Forinstance, 0.01 mole of YBa₂ Cu₃ O_(x) can be dissolved in 1 liter ofsolvent when isopropanol and isopropoxides are used as solvent andalkoxides, respectively.

It is desirable that the starting solution including alkoxides should berefluxed more than 10 minutes. The refluxing provides a chance fordispersed alkoxides to meet one another in solution and enables them toform a polynuclear complex or multialkoxide which have similarcoordination states as that of the oxides. An oxide powder having amulticomponent single phase can be obtained through hydrolysis of thesepolynuclear complexes or multialkoxides.

It is also preferable that refluxing during alkoxide synthesizing andmixing is carried out under a dry atmosphere so as not to hydrolyze samewith the moisture in air.

(2) Hydrolysis

During hydrolysis, a large amount of water is intolerable, becausewater-soluble hydroxides are formed from which copper or barium oxidesare generated during subsequent drying and heating processes.

Deionized water with high purity should be used, because dissociation ofcopper or barium from the complex compounds is caused by impurities,such as the chlorine ion, in the water and hence a change in thecompositions results. Furthermore, water diluted with the solvent isrecommended for obtaining a uniform hydrolysis reaction.

(3) Aging

The dried powder, which is obtained from the solution just afterhydrolysis, has a small particle size and a high surface activity. Thus,the powder has several problems including: (1) low crystallinity, (2)primary particles easily aggregate, (3) remarkable adsorption of wateron the particle surface in air, and (4) abnormal grain growth and bubbleformation during the sintering process. To avoid these problems, powdershaving diameters ranging from 0.01 to 0.1 μm can be prepared byrefluxing or holding the solution at a temperature close to the boilingpoint of the solvent after hydrolysis. A partially dried or condensedsolution after aging is close to a gel state, and in it, copper, bariumand yttrium exist mostly in the oxide form with a size of less than 1μm. The process for preparing these starting materials is called thesol-gel method.

(4) Drying

Available drying methods include keeping the suspension involved as fineparticles at a temperature lower than the boiling point of the solvent,or under reduced pressure for evaporating the solvent, and spraying thesuspension in the form of small droplets and drying same rapidly in hotair.

The powders, produced as described above, have a size of 1 μm or lessand a single phase for superconducting oxides. Thus, they can besintered at a temperature lower than that of the conventional processes.

The present invention will be described in detail in the followingexamples.

EXAMPLE 1

Metallic barium of 0.02 mole was added to 1 liter of isopropanol andrefluxed at 83° C. for 30 minutes. A colorless, transparent bariumisopropoxide dissolved in the isopropanol was obtained. Yttriumisopropoxide dissolved in 1 liter of isopropanol (concentration: 0.01mole/liter) at 70° C., was prepared by an exchange reaction of yttriumchloride and sodium isopropoxide and used just after preparation, wasadded to the barium isopropoxide solution and refluxed again for 30minutes. Thereafter, copper isopropoxide powder of 0.03 mole was addedto the resultant solution and refluxed again for 1 hour. Into thishomogeneous solution, 0.6 mole of pure water was added during refluxing.After refluxing for 2 hours, the solvent was evaporated at 70° C. underreduced pressure, and a powder was obtained.

The resultant powder sample was heated at 200° C. for 1 hour and thespecific surface area was measured by the BET method. The specificsurface area value corresponded to an average particle size of 0.005 μm.

The powder was pressed into pellets and heated at various temperaturesfor 1 hour in a furnace under an oxygen atmosphere, followed by slowcooling to room temperature. In FIG. 1, an X-ray diffraction pattern ofthe sample heated at 750° C. is shown. The sample shows a single phaseof YBa₂ Cu₃ O_(7-x). Porosity change was measured against treatmenttemperature (Curve A in FIG. 2). At 750° C., the porosity was about 10%and sintering of the sample was completed. The electrical resistance ofthe sample heated at 750° C. was measured by the DC four-probe techniqueand the temperature of zero resistance was about 90 K.

EXAMPLE 2

Powder yttrium, barium and copper isopropoxides of 0.01, 0.02 and 0.03mole, respectively, were added into 1 liter of isopropanol and refluxedwith stirring at 83° C. for 8 hours. The alkoxides were not perfectlydissolved into the solvent and black precipitates remained. Water of 0.6mole was added into the suspension while refluxing, and refluxing wascontinued for 2 hours. Thereafter, the solvent was evaporated at 70° C.under reduced pressure and a powder sample was obtained.

The crystal phase of the powder was identified by its X-ray diffractionpattern as a mixture of CuO, Y₂ O₃ and BaCO₃.

The powder was heated at 400° C. for 5 hours to evolve solvent andadsorbed water, and then the specific surface area was measured by theBET method. The average particle size was calculated from the surfacearea value to be 0.012 μm assuming spherical particles.

The powder was pressed into pellets and heated in a furnace at varioustemperatures in an oxygen atmosphere, followed by slow cooling to roomtemperature. Crystal phase of the samples was determined by the X-raydiffraction method. As a result, it was found that 750° C. heating gavemixed phase of CuO or BaCO₃ and YBa₂ Cu₃ O_(7-x). On the other hand,900° C. heating gave a single phase of YBa₂ Cu₃ O_(7-x). Porosity changeafter heating was measured against treatment temperatures (Curve B inFIG. 2) by a mercury injection method. The result indicates thatshrinkage began at 600° and finished at 900° C. The porosity after 900°C. heating was 11%. Resistance was measured by a conventional four-probemethod. The zero resistance temperature after 900° C. heating was about88 K.

EXAMPLE 3

Powder was produced by a similar process as that described in example 1except for the refluxing time after hydrolysis. After 24 hoursrefluxing, powder was heated at 200° C. for 1 hour and the specificsurface area was measured by the BET method. The average particle sizecorresponds to the surface area value and was 0.09 μm. Thus, bycontrolling the reflux time, particles within 0.05 μm to 0.09 μm can beobtained by this technique.

EXAMPLE 4

Instead of yttrium isopropoxide, ytterbium or holmium isopropoxides wereused and a similar experiment to example 1 was carried out. As a result,a LnBa₂ Cu₃ O_(7-x) (Ln is Ho or Yb) single phase was obtained byheating at 750° C. Both of the samples showed about a 10% porositity anda T_(c) of around 90 K.

As shown in this example, a system containing a lanthanoid also providespowders with a small particle size and a single phase of asuperconducting oxide. Dense superconducting sintered oxides which showthe superconducting state above the boiling temperature of nitrogen canbe obtained by treatment at a relatively low temperature, 750° C.

A very fine powder can be prepared by the sol-gel method of theinvention. The powder, however, has some problems, such as adsorption ofwater in air and bubbling during the sintering process. These problemsof the sol-gel powder can be improved by mixing them withsuperconducting oxide crystals of micron or submicron particle sizewhich were prepared by treatment of commercially available reagents orcommercially available oxide powders at high temperatures.

As commercially available powders, a mixture of Y₂ O₃, CuO, BaCO₃, etc.mixed in a predetermined ratio, can be used for mixing with the sol-gelpowders. Also, powder in the superconducting phase which was preparedfrom commercially available reagents by sintering at 900°-950° C. in anoxygen atmosphere, followed by slow cooling to room temperature, can beused for mixing with the sol-gel powders.

EXAMPLE 5

First of all, the properties of the powder made by the sol-gel techniqueand a mixture of powder of commercially available reagents are comparedto each other.

A copper ethoxide (Cu(OC₂ H₅)₂) powder, barium butoxide (Ba(OC₄ H₉)₂) inbutanol and yttrium butoxide (Y(OC₄ H₉)₃) in butanol were weighed tomake a mixture having a Y:Ba:Cu= 1:2:3: mole ratio and were mixed with1-butanol as a solvent. The resultant solution was refluxed at about120° C. for 10 hours and distilled water of 5 to 10 times the totalmolar amount of Y, Ba and Cu was added. Then, the solution was refluxedfor about 10 hours to complete hydrolysis and fine particles weregenerated in the solution. The solution including the fine particles waskept at about 120° C. to evaporate the solvent. Dry sol-gel powder wasobtained and stored in a dry box. On the other hand, commerciallyavailable reagents of Y₂ O₃, BaCO₃ and CuO were weighed to make amixture having a 1:2:3 mole ratio and mixed.

For comparison, the sol-gel powder and the mixture of the commericallyavailable reagents were heated at a temperature ranging from 250° to900° C. and specific surface areas of the samples were measured by theBET method (FIG. 3).

In the sol-gel powder (curve C in FIG. 3), a decrease in specificsurface area was already observed at around 300° C., which indicates thestart of sintering at this temperature. By contrast, the mixture ofcommerically available reagents (curve D in FIG. 3) showed a gradualdecrease in surface area at above 900° C. (indicating the progress ofthe sintering). The heat treated samples were examined by X-raydiffraction. In the sol-gel powder, some peaks which can be assigned toa superconducting phase were observed in the sample heated at 750° C.,while in the commerically available powder mixture, the peaks of asuperconducting phase were observed in the sample heated at or higherthan 900° C.

The commerically available powder mixture was sintered at 950° C. in anoxygen atmosphere and slowly cooled (hereinafter call "commerciallyavailable sintered powder"). The sol-gel powder was mixed with thecommercially available sintered powder in a weight ratio of 1:1. Theresultant mixture of powder was sintered at various temperatures of400°, 500°, 750°, 800° and 950° C., and the X-ray diffraction patternswere measured. The resulting X-ray diffraction patterns of the samplesheated at 750°, 800° and 950° C. are shown in FIGS. 4A, 4B and 4C.

Peaks which can be assigned to the superconducting phase (shown by anarrow in the diffraction pattern of the powder sintered at 750° C.;other peaks are assigned to BaCO₃) appeared in the samples heated at orabove 750° C. In the 800° C. sample, a single phase of YBa₂ Cu₃ O_(7-x)was observed and the diffraction pattern was almost the same in the 950°C. sample. Thus, it can be said that the superconducting phase appearedat 800° C.

The above results indicate that the technique of mixing the sol-gelpowder with the commercially available powder can improve theabove-mentioned problems, such as adsorption of water and so on, andalso showed that the superconducting phase can appear at a temperaturelower than that of the conventional technique.

Powders prepared by mixing the sol-gel powder with the commerciallyavailable sintered powders in the ratios of 1:1, 1:05 and 0.5:1 werepressed into pellets and sintered. The electrical resistance versustemperature curves of these samples were measured by a conventionalfour-probe technique. They showed zero resistance temperatures at about80 to 90 K.

A method for preventing reaction of a superconducting oxide film and asubstrate during the sintering process will be described below. A bufferlayer is formed on a substrate, and a superconducting oxide film isformed on it.

It is preferable to use zirconia for the buffer layer, because it has alow reactivity at high temperature. Zirconia, which has a transitionpoint at about 1100° C., shows a large volume change when it istransformed from a tetragonal (high-temperature form) to a rhombic phase(low-temperature form). Thus, heating higher than the phase transitionpoint often causes breakage of the film during the course of temperaturerise and fall. Therefore, it is more preferable to use zirconiacontaining yttrium, calcium etc. of 20% or less, because addition ofthese elements stabilizes a cubic phase in the low temperature regionand suppresses the tetragonal-rhombic phase transition. In order toprevent reaction of the superconducting film and the substrate, morethan a 0.1 μm thickness of zirconia buffer is necessary. A method forforming a zirconia buffer layer will be described in detail below.

Procedures for forming a zirconia film include: (1) washing a substrate,(2) preparing a sol, (3) coating, (4) drying, and (5) sintering.

(1) Washing a Substrate

Substrates such as silicon, SiO₂ glasses, alumina etc. are washed withan acid, pure water, an organic solvent, and dried. Acids, for example,nitric acid and hydrofluoric acid can be used for removing foreignmattter from a substrate surface. Sufficient drying of a substrate isrequired, because water or something like that on the surface preventsthe formation of homogeneous buffer film.

(2) Preparing a Sol

Desirable conditions for preparing a sol depend on the main startingmaterials of the film. Cases where zirconia alkoxides and organometallicacids are used respectively are described. Zirconium alkoxides: thezirconium alkoxide is diluted with a solvent containing ahydrolysis-suppresser. Often compounds of elements for stabilizing acubic phase of zirconia, such as yttrium or calcium, are added in asolvent. Zirconium propoxide and butoxide are recommended since they aremildly hydrolyzed (when compared with the methoxide or ethoxide). Thehydrolysis-suppressor is used to suppress the rate of hydrolysis ofhigh-activity alkoxides and to make a homogeneous sol. The suppressorsinclude a carboxylic acid, such as acetic acid, and polyhydric alcohols,such as ethyleneglycol, and acetylacetone, and so on. More than 0.01mole of suppressor is needed for 1 mole of alkoxide. As the solvent,organic materials which have boiling points of 200° C. or below arepreferable, because they can be evaporated easily. The ratio of thesolvent is more than 1 mole and less than 1000 mole for 1 mole ofalkoxide. Alkoxides are also preferable for the starting reagents ofyttrium and calcium. Addition of water containing an acid catalyst, suchas diluted hydrochloric acid, into the sol is useful to keep theproperties of the sol for a long period of time because the additionmoderates a polycondensation reaction. Organometallic acid: for example,when zirconium acetate is used, a hydrolysis-suppresser or even water isnot necessary. The other conditions are the same as in the case ofzirconium alkoxide.

(3) Coating

Sol is uniformly coated on a clean substrate. Adequate coating methodsinclude the dipping method, spin-coating method, spray coating method,and die coating method. The thickness of the film can be controlled bychoosing the concentration of raw materials, speed of pulling up in thedipping technique, rotation speed in the spin-coating technique, sprayrate in the spray coating technique, and pulling speed in the diecoating technique. Since formation of a thick film by one-step coatingresults in cracking of the film during the drying process, the thicknessof the film formed by one-step coating must be restricted within 0.5 μm.

(4) Drying

The sol films coated on substrates are put in air. Then, hydrolysis andevaporation of the solvent proceed and dry films can be obtained. Dryingshould be done in an atmosphere where the humidity and vapor pressure iscontrolled to be constant.

(5) Sintering

A ZrO₂ film on a substrate can be obtained by sintering at a temperatureof 500° C. or higher. The sintering atmosphere desirably containsoxygen.

The film fabricated in the above-mentioned manner is a dense andhomogeneous film with a fine rhombic or cubic zirconia phase of 0.01 to0.5 μm in thickness. A thick zirconia film can be produced by repeatingthe sol coating and drying, followed by sintering or by repeating allprocedures from sol coating to sintering.

Various methods can be applied to form a superconducting oxide film onthe buffer layer. The liquid-phase technique gave an orientedsuperconducting film on a zirconia-coated substrate. Particularly, ahomogeneous solution with high concentration can be prepared bydissolving alkoxide starting materials in a polyhydric alcohol solventor a mixed solvent of a polyhydric alcohol and acetic acid. The coatedfilm made from this solution by the dipping, spin coating, or spraycoating techniques, followed by drying and sintering processes, ischaracterized by an excellent, smooth surface.

EXAMPLE 6

A silicon wafer with a polished surface was washed with hydrofluoricacid and used as a substrate. A zirconium alkoxide, Zr(n-OC₄ H₉)₄, wasdiluted with isopropanol and mixed with a small amount of acetic acid(starting sol). The substrate was dipped into the starting sol andpulled up at a certain rate. The sol coating on the substate washydrolyzed and dried in air. The sol coated substrate was sintered in anelectrical furnace at 500° C. in an oxygen atmosphere. The procedurefrom dipping to sintering was repeated several times and azirconia-coated Si substrate was prepared. Meanwhile, 0.03 mole ofCu(n-OC₄ H₉)₂, 0.02 mole of Ba(n-OC₄ H₉)₂, and 0.01 mole of Y(n-OC₄ H₉)₃were dissolved in n-butanol, hydrolyzed, and the solvent evaporated. Theobtained sol-gel Y-Ba-Cu-O powder was mixed with ethyleneglycol to makea paste, and printed on a zirconia-coated Si substrate by thescreen-printing technique. After drying, the film was sintered at 900°C.

The zirconia film was examined by X-ray diffraction and by a scanningelectron microscope (SEM). The thickness of the film was measured by thecontact method. The sintered Y-Ba-Cu-O coated film was examined by X-raydiffraction, and the resistance of the film was measured by the DCfour-probe technique.

The X-ray diffraction pattern of the zirconia film on Si substrate isshown in FIG. 5. Since the intensities of the diffraction peaks are lowand their half-width values are large, the zirconia coating seems tohave the form of microcrystals. High homogeneity was achieved in thezirconia film, and the surface of the zirconia film was smooth asobserved by SEM.

In this experiment, one dipping gave a zirconia film of 0.05 μm inthickness. A 2.5 μm thick film was obtained by repeating the process 50times.

The X-ray diffraction pattern of the Y-ba-Cu-O sintered film on azirconia film of 2.1 μm in thickness is shown in FIG. 6. The film was asingle phase of YBa₂ Cu₃ O_(7-x). Since the (002), (003), (005) and(006) peaks are strong, it can be said that the film is c-axis-orientedperpendicular to the substrate surface.

In FIG. 7, the electrical resistance versus temperature curve for thesuperconducting film is shown. The onset temperature and the zeroresistance temperature were 91 K and 86 K, respectively. The transitionwidth was 5 K. These good results are attributed to complete connectionin the a-b planes, which function as the superconducting path in thefilm due to the c-axis orientation and the high density of the film.

EXAMPLE 7

A pure silica glass plate after washing with hydrofluoric acid was usedfor a substrate. Zr(n-OC₄ H₉)₄ of 0.9 mole and Y(n-OC₄ H₉)₃ of 0.1 molewere mixed in isopropanol as solvent and a small amount of acetic acidwas added in to make a starting sol. The sol film was fabricated by thespin coating technique, hydrolyzed, and dried in air. The film wassintered in the same manner as in example 5 and the procedure fromcoating to sintering was repeated 50 times. Cu(OC₂ H₅)₂ of 0.03 mole,Ba(n-OC₄ H₉)₂ of 0.02 mole, and Y(n-OC₄ H₉)₃ of 0.01 mole were dilutedwith ethyleneglycol containing acetic acid and a transparent solutionwas prepared. The film was made by the spin coating technique, wasdried, and was sintered at 900° C. The Y-Ba-Cu-O superconducting filmwas obtained on the Y-stabilized zirconia buffer layer. The procedurefrom spin coating to sintering was repeated 50 times.

The yttrium-added zirconia film was confirmed to be a dense andhomogeneous film by SEM observation, and it is composed of cubiczirconia as determined from its X-ray diffraction pattern. The cubiczirconia was stabilized by addition of yttrium down to room temperature.The film had a thickness of 1.6 μm.

The Y-Ba-Cu-O film had a 1.0 μm thickness and a smooth surface. The filmwas c-axis-oriented YBa₂ Cu₃ O_(7-x) as determined from its X-raydiffraction pattern. The film showed an onset temperature of 90 K and azero resistance temperature of 85 K.

EXAMPLE 8

A silicon wafer with a polished surface was used for a substrate. Asolution of Zr(n-OC₄ H₉)₄ was prepared in the same manner as in example6 and was sprayed on the substrate heated at 400° C., and the solventdried rapidly. The procedure from spraying to drying was repeated 20times. The zirconia film was obtained by sintering the film at 900° C.

A solution of the Y-Ba-Cu system was prepared in the same manner as inthe example 7 and was then sprayed on the zirconia-coated substrate,heated at 400° C., and dried. Then, a Y-Ba-Cu-O superconducting film wasprepared by sintering at 900° C.

The zirconia film and the sintered Y-Ba-Cu-O film were examined by X-raydiffraction and by a scanning electron microscope (SEM), and thethickness of the films were measured by the contact method. Theresistance of the Y-Ba-Cu-O film was measured by the DC four-probetechnique.

The zirconia film made by the above-mentioned process was a rhombicphase and had a thickness of 1.5 μm. The sintered Y-Ba-Cu-O film wascomposed of an oriented (YBa₂ -Cu₃)_(7-x) crystal and had a thickness of0.9±0.2 μm. The film showed an onset temperature of 90 K and a zeroresistance temperature of 84 K.

EXAMPLE 9

A silica glass optical fiber was drawn from a preform in a carbonresistance furnace and immediately coated with the starting sol ofzirconia (prepared in the same manner as in example 6) using the diecoating technique. The coated fiber was sintered at 500° C. in a furnaceinstalled just under the drawing furnace. Subsequently, the resultantzirconia-coated fiber was coated with a Y-Ba-Cu alkoxide mixed solutionby the die coating technique, followed by sintering at 900° C. to make aoxide-coated optical fiber.

The oxide coating layer was removed and identified as the YBa₂ Cu₃O_(7-x) phase by the powder X-ray diffraction method.

As shown in this example, a superconducting oxide-coated optical fibercan be produced by the present invention, which suggests the possibilityof a power supply to an optical communication system using asuperconducting line. Although, in the above-mentioned examples, a solsolution, which was made from zirconium alkoxide by hydrolysis, was usedfor forming the buffer layer, the buffer layer can also be produced by apyrolysis technique using an organic acid salt, or by a chemical orphysical vapor deposition method, and so on. Moreover, although, in theexamples, a silicon wafer, a silica glass plate and an optical fiberwere used for the substrates, the present invention can be applied tosubstrates of various shapes and materials, for example, a cylinder or atape-shaped substrate, and alumina or InP or other materials.

A method for fabricating a c-axis-oriented superconducting film will bedescribed below.

In making an Ln-Ae-Cu-O (Ln: yttrium or a lanthanoid, Ae: at least onealkali earth metal) or a Bi-Ae-Cu-O super-conducting oxide, the use ofstarting materials within a particular composition region can enhancethe degree of c-axis orientation.

EXAMPLE 10

A copper alkoxide, Cu(OC₂ H₅)₂, a barium alkoxide, Ba(OC₄ H₉)₂, and ayttrium alkoxide, Y(OC₄ H₉)₃ were mixed together in a ratio so as toprovide an oxide mixture composed of 66 mole % Cu, 27 mole % Ba, and 7mole % Y, and were added into n-butanol as a solvent. The resultantmixed solution was refluxed for 20 hours and pure water of 5 to 10 timesthe total of Cu, Ba and Y was titrated into the solution with ultrasonicvibration. The solution was refluxed again for 10 hours to complete thehydrolysis reaction. Then, the homogeneous solution was partially driedto increase the viscosity and make an appropriate paste for paintingonto a substrate. A part of the solution was dried completely andsubjected to a composition analysis.

The paste was painted on a yttrium-stabilized zirconia (YSZ) substrateand heated at 950° C. for 30 minutes. Then, the sample was slowly cooledto 600° C. at a rate of 1° C./min, kept at 600° C. for 10 hours, andslowly cooled again to room temperature under flowing oxygen gas.

The X-ray diffraction pattern of the resultant film on YSZ is shown inFIG. 8. Since the (002), (003), (004), (005), (006) and (007) peaks arevery strong, it can be said that the film was c-axis-orientedperpendicular to the substrate.

FIG. 9 shows electrical resistance of the film as measured by aconventional four-probe technique. The onset temperature was 95 K andthe zero resistance temperature was 90 K. The results are comparable tothe T_(c) value of bulk samples.

The composition of the film was determined by X-ray fluorescenceanalysis. The change in the composition of the film upon heating isshown in FIG. 10, wherein the abscissa represents the treatmenttemperature, while the ordinate represents mole percentage.

The arrows at the right side of the figure show the mole percentage of asuperconducting phase of YBa₂ Cu₃ O_(7-x) (Y: 16.7 mole %, Ba: 33.3 mole%, Cu: 50.05 mole %).

Although the starting composition of the compound was chosen to belocated on the Cu-rich and Y-poor side, the Cu content decreasedremarkably during the heat treatment when the temperature is increased.Therefore, consequently, Ba and Y contents increased relatively andapproached a 1-2-3 composition as indicated by the arrows. As a result,it was found that the composition deviation of the starting compoundtoward the Cu-rich side from the 1-2-3 composition is naturallycorrected by heating, and a Cu-rich starting composition can provide astrongly c-axis-oriented film on a ceramic substrate. The reason why thec-axis orientation can be achieved from the Cu-rich starting compositionis not yet clear. However, the melting points of CuO, BaO and Y₂ O₃ areknown to be 1232° C., 1923° C. and 2440° C., respectively. Therefore, itcan be expected that the element which shows the lowest melting point,CuO, fist reacts at a low temperature and then a rearrangement of atomsresults in the orientation.

Compositions of the starting Y-Ba-Cu-O compounds (made from alkoxides)are plotted in a ternary system graph in FIG. 11. Sintering was carriedout at 950° C. for 30 minutes.

Circles in this figure show a highly oriented sample as shown in theX-ray diffraction pattern in FIG. 8. Triangles show a partially orientedsample as shown in the X-ray diffraction pattern in FIG. 12. The crossesstand for a non-oriented sample. The 1-2-3 starting composition, whichis shown as closed circles in FIG. 11, gave a non-oriented sample. Itcan be confirmed from FIG. 11 that suitable starting compositions forthe orientation are those having less than 16.6 mole % Y and more than50 mole % Cu.

The above-mentioned phenomenon is expected to be notable when a film isformed from fine oxide particles having less than submicron size, suchparticles being made by the sol-gel method as in this example. Thesmaller the particle size, the higher the activity of the particles, andhence the lower the temperature at which migration of atoms becomespossible. In order to demonstrate this hypothesis through an experiment,the same orientation experiment by heating was carried out usingcommercially available reagents, such as Y₂ O₃, BaCO₃ and CuO, of largeparticle sizes (1 to several μm) as starting materials. Since a highertemperature is necessary for achieving an orientation in such reagentshaving large particle sizes, the temperature was set at 1000° C. for 30minutes. In the preliminary experiments using commercially availablereagents, it was confirmed that no clear orientation can be observedwhen the heating is lower than 1000° C., and when higher than 1000° C.,reaction of the superconduction oxide film with the substrate is sosevere that the superconductor is damaged. In FIG. 13, the startingcompositions of commercially available reagents are shown with thedegree of c-axis orientation. Triangles and crosses show partiallyoriented and non-oriented samples, respectively.

The region which shows orientation is slightly narrower than that inFIG. 11 (in the case of sol-gel powder) and the degree of orientation islower. Consequently, fine particles prepared by the sol-gel method aremore suitable than the commercially available reagents in terms oforientation.

EXAMPLE 11

A copper alkoxide, Cu(OC₂ H₅)₂, a barium alkoxide, Ba(OC₄ H₉)₂, andneodymium alkoxide, Nd(OC₄ H₉)₃ were mixed together in such a ratio aswill give a Cu-rich and Nd-poor composition oxide (Cu: 60.2 mole %, Ba:30.8 mole %, Nd: 9.0 mole %) and were added into n-butanol as a solvent.The same procedures of reflux and hydrolysis as in example 10 werecarried out. The resultant solution was condensed to have a suitableviscosity for screen printing or spray coating techniques. The filmswere formed on YSZ substrates by these techniques. The film on a YSZsubstrate was heated at 950° C. for 30 minutes in flowing oxygen gas,and then slowly cooled to room temperature. The X-ray diffractionpattern for the thus-obtained Nd-Ba-Cu-O film is shown in FIG. 14.

Although no peaks assigned to (002) and (004) were observed, other peakscorresponding to (OON) (N=3, 5, 6, 7) appeared with high intensities.Thus, it was also confirmed that, for a Nd-Ba-Cu-O system, c-axisorientation can be performed when a starting compound with a Cu-rich andNd-poor composition is used.

EXAMPLE 12

A Y-Nd-Ba-Cu-O superconducting film was produced from alkoxides asstarting materials on a YSZ substrate in a manner similar to examples 10and 11. The starting composition was 7.8 mole % Y, 5.3 mole % Nd, 17.9mole % Ba, and 69 mole % Cu; i.e., a Cu-rich and Y, Nd-poor composition.The X-ray diffraction pattern for the Y-Nd-Ba-Cu-O superconducting filmsample is shown in FIG. 15.

Peaks peculiar to c-axis orientation were observed. Furthermore, sincethe ionic radii of Y and Nd are different from each other (Y: 1.06 Å andNd: 1.15 Å), the lattice constants are also different (lattice constant:Y<Nd, 2Θ: Y>Nd), and the peaks corresponding to (005), (006) and (007)for Y and Nd appeared separately. In the figure, circles and stars standfor the peaks for Y and Nd, respectively.

Consequently, it is found that film of an Ln-Ba-Cu-O (Ln: lanthanoid)system and a mixed system can also be c-axis-oriented when a suitablestarting composition is chosen.

Although the shape of the substrate in the examples is plate-like, thepresent invention can also be applied to variously shaped substrates.For example, since ceramics, such as YSZ, are known to showsuperplasticity, fiber or other shapes can be produced.

EXAMPLE 13

A Y-Ba-Cu-O powder was prepared by the sol-gel method and mixed withethylene glycol to make a paste. The paste was painted on an Al₂ O₃substrate and heated at 900°-950° C. for 10-30 minutes to make asuperconducting film. Since an Al₂ O₃ substrate generally tends to showa slightly higher reactivity with a superconducting material at hightemperature than a YSZ substrate, the heating temperature for films onAl₂ O₃ substrates must be a little lower than that for YSZ substrates.Nevertheless, a c-axis-oriented film was obtained from a Cu-rich andY-poor starting composition as shown in examples 10 to 12. Therefore, itcan be said that basically there is no limitation on the selection ofsubstrates to form oriented superconducting oxides thereon.

The reactivity of substrates with a superconducting material at a hightemperature, of course, depends on the material of the substrate.Through these experiments, however, it was confirmed by the inventorsthat it is also possible to form a c-axis-oriented film on a bufferlayer, such as CuO, Y₂ O₃, ZrO₂ etc., which can be prepared by, forexample, using the sol-gel method.

A c-axis-oriented superconducting oxide film in the Bi-Ae-Cu-O system(where Ae is at least one alkali earth metal) can also be made in thesame manner as described above.

EXAMPLE 14

Commerically available reagents of Bi₂ O₃, SrCO₃ CaCO₃, and CuO withaverage particle sizes of 1 μm or more were mixed and added intoethylene glycol, and mixed again sufficiently to make a paste forpainting. The paste was painted on a YSZ or an Al₂ O₃ substrate byscreen printing using a 300 mesh screen. After adequate drying, the filmon the substrate was sintered at 900° C. A Bi-Sr-Ca-Cu-O film of 10 μmin thickness was obtained.

Crystal structure and the degree of orientation of the films weremeasured by X-ray diffraction. Electrical resistance of the films wasmeasured by the DC four-probe technique.

Starting compositions in the Bi-(Sr, Ca)-Cu-O system (SrO/CaO+SrO)=0.5)are shown in FIG. 16. Closed and open circles show the samples ofsuperconducting phase, and crosses show non-superconducting samples. Theclosed circles stand for c-axis-oriented superconducting samples. Thus,in FIG. 16, the area surrounded by A-B-C-D lines (Bi/(Cu+Sr+Ca+Bi)=4 to40 mole %, (Sr+Ca)/(Cu+Sr+Ca+Bi)=15 to 70 mole % Cu/(Cu+Sr+Ca+Bi)=24 to64 mole %) is the region where a superconducting phase of Bi₂ (Sr, Ca)₃Cu₂ O_(x) appeared. Particularly, the region surrounded by A-B-E-D lines(Bi/(Cu+Sr+Ca+Bi)=5 to 40 mole %, (Sr+Ca)/(Cu+Sr+Ca+Bi)=15 to 40 mole %,Cu/(Cu+Sr+Ca+Bi)=24 to 64 mole %) shows the area where c-axis-orientedfilm samples were obtained. When the ratio of SrO/(CaO+SrO)=0.25 to0.75, c-axis-oriented samples were also obtained.

The X-ray diffraction pattern of the film made from the startingcomposition in the region surrounded by A-B-E-D lines is shown in FIG.17. Strong diffraction peaks, which are assigned to OON) of Bi₂ (Sr,Ca)₃ Cu₂ O_(x) (c-axis length: 30.6Å), are observed in this pattern.This indicates c-axis orientation perpendicular to the substrate.

An electrical resistance versus temperature curve for the orientedsample is shown in FIG. 18. The film showed a sharp transition with anonset temperature of 88 K and a zero resistance temperature of 83 K.

Although the reason why c-axis orientation occurs in the compositionregion surrounded by A-B-E-D lines in FIG. 16 is not yet clear, it canbe presumed that partial melting is also attributed to the orientation,because the region is located in the Cu and Bi-rich side (elementshaving low melting points in the system).

EXAMPLE 15

Alkoxides were used as the starting materials to make a superconductingfilm as in example 14. Bi(OC₄ H₉)₃, Sr(OC₄ H₉)₂, Ca(OC₃ H₇)₂ and Cu(OC₃H₇)₂, in respective n-butanol solution were mixed and hydrolyzed,followed by refluxing for 10 hours to form fine oxide particles. Thesolvent was evaporated to make a paste. The paste was screen-printed ona YSZ substrate and after drying was sintered at 850° C. A Bi-Sr-Ca-Cu-Osuperconducting oxide film was obtained.

In using alkoxides as starting materials, the superconducting phase ofBi₂ (Sr, Ca)₃ Cu₂ O_(x) also appeared in the composition regionsurrounded by A-B-C-D lines and c-axis orientation was also observed inthe samples whose compositions belong within the region surrounded byA-B-E-D lines in FIG. 16. The X-ray diffraction pattern for the orientedfilm whose starting composition is within the region surrounded byA-B-E-D lines is shown in FIG. 19. The degree of orientation of the filmis higher than that of the sample in FIG. 17, that is, of the sampleprepared by using commercially available reagents. The onesettemperature and the zero resistance temperature were 88 K and 87 K,respectively. The transition from the normal to the superconductingphase was very sharp.

Thus, using alkoxides as the starting materials, high quality films witha high degree of orientation can be produced as compared with usingcommercially available reagents.

EXAMPLE 16

A superconducting film in the Bi-based system was formed on azirconia-coated silicon substrate.

A silicon wafer with a polished surface was used for the substrate. Astarting sol for the zirconia coating was prepared from a zirconiumalkoxide, Zr(n-OC₄ H₉)₄, diluted with isopropanol. A small amount ofacetic acid was also added to the starting sol. The silicon wafer wasdipped into the sol and pulled up at a constant rate. The coatedsubstrate was dried in air and sintered at 900° C. in an oxygenatmosphere. The procedures from dipping to sintering were repeated 50times to obtain a zirconia-coated silicon substrate. A Bi-Sr-Ca-Cu-Opaste, in the composition region A-B-E-D, was prepared by alkoxidehydrolysis and was screen-printed on the zirconia-coated siliconsubstrate, and sintered at 875° C. in an oxygen atmosphere.

The crystal structures of the zirconia-coated substrate and the sinteredBi-Sr-Ca-Cu-O film were identified by X-ray diffraction and theresistance versus temperature curve of the oxide film was measured bythe DC four-probe technique.

The surface of the zirconia coating was very smooth by opticalobservation and the film had a high homogeneity. The film thickness wasmeasured by X-ray diffraction and was 2.5 μm.

The X-ray diffraction pattern of the sintered Bi-Sr-Ca-Cu-O film showedformation of a c-axis-oriented Bi₂ (Sr, Ca)₃ Cu₂ O_(x) phase. The onsetand zero resistance temperatures of the film were 88 K and 80 K,respectively, with a transition temperature width of 8 K.

Although the screen printing technique was used in the example, varioustechniques, such as spray coating etc., can be applied to make asuperconducting film. And, while in the example, zirconia-coated siliconwas used for the substrate, materials for the substrates and bufferlayer are not limited so long as they have a low reactivity with thesuperconducting oxide formed in the sintering process.

Processes for preparing a c-axis-oriented bulk sample will be describedbelow. By using a pressing technique on the superconducting oxides, abulk sample which is c-axis-oriented parallel to the pressing directioncan be produced.

EXAMPLE 17

Reagents Y₂ O₃, BaCO₃ and CuO were mixed to make two types of compoundswith different compositions; one is Y:Ba:Cu=1:2:3 (called a 1-2-3composition) of Y=16.7 mole %, Ba=33.3 mole %, and Cu=50.0 mole %, andthe other is a composition of Y=8 mole %, Ba=30 mole %, and Cu=62 mole %(called an orientation composition). The mixtures were pre-sintered at950° C. for 5 hours in an oxygen atmosphere and ground to make powdersamples.

X-ray diffraction patterns for samples of the orientation compositionand the 1-2-3 composition are shown in FIG. 20. The degree oforientation can be estimated roughly by comparing intensities of the(OON) peaks (N is an integer) and the other peaks, for example, (006),(110), and (103). The peak ratio of R=(006)/(110)(103) were 1.06 for thesample of the orientation composition and 0.52 for that of the 1-2-3composition.

The two kinds of composition powders were pressed into pellets withvarious pressures and examined by X-ray diffraction. The direction ofthe pressure is parallel with the c-axis as shown in FIG. 21. In FIG.22, the degree of the orientation (estimated by the (006)/(110)(103)peak ratio) are plotted against applied pressure. Curve E shows the peakratios for the pellet samples made from the powder with the orientationcomposition, and curve F shows those for the sample from the powder withthe 1-2-3 composition. The powder with the orientation composition showshigher orientation than that with the 1-2-3 composition.

The powder sample with the orientation composition contains a largeamount of plate-like crystals which can be easily oriented by pressing.Thus, after pressing, the sample shows an excellent degree oforientation.

As shown schematically in FIG. 21, since a unit cell 1 with a, b, and caxes (a=3.8181 Å, b=3.888 Å, c=11.688 Å) forms a plate-like crystal 2,these plate-like crystals are easily arranged by pressing to stabilizethem against applied pressure.

EXAMPLE 18

In the Y-Ba-Cu-O system, compositions were changed to attain a highorientation by pressing. Mixtures of the reagents were sintered at 950°C. in an oxygen atmosphere (the same conditions as in the example 17).Pellets were made by pressing at 19000 kg/cm². The results are shown inFIG. 23. In the ternary system graph, open circles stand for the sampleswhich show the same order of orientation as that of the orientationcomposition, and closed circles stand for the samples which show thesame order or less of orientation as that of the 1-2-3 composition. Thecomposition region of open circles (high orientation region) agrees wellwith that in the oriented film (see FIG. 11). The high orientationregion is Y/(Y+Ba+Cu)<16.6 mole % and Cu/(Y+Ba+Cu)>50 mole %, which islocated on the Y-poor and Cu-rich side.

EXAMPLE 19

Powder samples with the 1-2-3 composition were pressed into pellets atvarious pressures and were resintered at 875° C. for 40 hours in anoxygen atmosphere. The pellets were checked by X-ray diffraction. Thepeak ratios of (006)/(110)(103) in the 1-2-3 composition samples wereplotted against pressures used in the pressing process in FIG. 24. CurveG shows the degree of orientation for the samples subjected to onlypressing, and curve H those for the samples subjected to pressing andre-sintering at 875° C. No noticeable change in the degree oforientation was observed after resintering at that temperature. Althoughthe results in the samples with the orientation composition are notshown here, almost the same results are obtained; i.e., no decrease wasobserved after re-sintering at 875° C.

Electrical resistance of the re-sintered pellet samples were measured bya conventional four-probe technique. The onset temperature was 93 K, andthe zero resistance temperature was 90 K.

In this example, re-sintering was carried out to ensure mechanicalstrength against thermal shock in this measurement.

EXAMPLE 20

A copper alkoxide, Cu(i-OC₃ H₇)₂ in isopropanol, a barium alkoxide,Ba(n-OC₄ H₉)₂ in butanol, and a yttrium alkoxide, Y(n-OC₄ H₉)₃ inbutanol were mixed in such a ratio as to give an oxide with thecomposition, 60 mole % Cu, 30 mole % Ba, and 10 mole % Y.

The resultant mixed solution was refluxed for 20 hours, and pure waterin an amount of about 5 to 20 times the total molar amount of Cu, Ba andY was titrated into the solution with ultrasonic vibration. Then, thesolution was refluxed again for about 10 hours to ensure completehydrolysis. The solvent in the solution was evaporated at 120°-150° C.for 2 or 3 days. The dried sample was sintered at 900° C. for 5 hours inan oxygen atmosphere.

The sintered sample was ground into powder, pressed, and examined byX-ray diffraction. As a result, c-axis oriented samples ofR=(006)/(110)(103)=from 1.55 to 4 were obtained, which degree oforientation was higher than that of the sample made from commerciallyavailable reagents (R=1.06).

An X-ray diffraction pattern for the powder sample of R =4 obtained inthe above-mentioned manner is shown in FIG. 25.

EXAMPLE 21

Reagents Bi₂ O₃, SrCO₃, CaCO₃, and CuO were mixed to make variouscompositions, respectively shown by open circles, closed circles andcrosses in FIG. 16, and pre-sintered at 875° C. for 10 hours in aplatinum boat in an oxygen atmosphere. The sintered samples were groundinto powders. Then, using a pressing plate, the powders were pressed onthe sample plate by a vertical pressure of 1 kg/cm² or less and X-raydiffraction patterns (CuKa) were measured.

The compositions in the region surrounded by B-C-E lines in FIG. 16showed superconducting phases but did not show a high orientation.

An X-ray diffraction pattern for the sample of BiSrCaCu₃ O_(x) is shownin FIG. 26. In this diffraction pattern, (OON) peaks show higherintensities than the other peaks even at a pressure of 1 kg/cm² or less.Thus, the composition BiSrCaCu₃ O_(x) has a tendency to be highly c-axisoriented. In this diffraction pattern, the strong peak appeared at2Θ=5.8°. The 5.8° peak is commonly observed in compositions within theregion surrounded by A-B-E-D lines. The samples which are shown asclosed circles in FIG. 16, the composition region surrounded by A-B-E-Dlines, also show superconductivity and a high orientation in making anoriented bulk sample by pressing. However, in the compositions indicatedby crosses no peak appeared at 2Θ=5.8°. Hence these compositions are notin a superconducting phase.

In the cases of composition SrO:CaO=1.3 and SrO:CaO=3:1, the obtainedresults were similar to that described above.

An adequate amount of propylene glycol was added to the mixed and notpre-sintered powders having various compositions shown in FIG. 16 tomake pastes. Resultant pastes were coated on YSZ substrates and examinedby X-ray diffraction. As shown in example 14, the samples withcompositions in the region surrounded by A-B-E-D lines showed a highorientation when films were made from them. Accordingly, it can be saidthat the compositions surrounded by A-B-D-E lines, compositions whichare Cu or Bi-rich, are suitable for making a c-axis oriented sample inboth film and bulk form.

EXAMPLE 22

Reagents Bi₂ O₃, SrCO₃, CaCO₃, and CuO were mixed in a mole ratio of Bi₂Sr₁.5 Ca₁.5 Cu₂ O_(x) and pre-sintered at 875° C. for 10 hours in aplatinum boat in an oxygen atmosphere. The samples were ground intopowders and pressed into pellets at various pressures to examinepressure dependency of the degree of the orientation.

X-ray diffraction patterns for these samples are shown in FIG. 27. At apressure of 1 kg/cm² or less, no orientation was observed. At 190kg/cm², c-axis orientation was observed, and at 950 kg/cm² the degree ofthe orientation was enhanced.

Peak ratios of (008)(2Θ=23.2°)/2Θ=27.5° are plotted against pressure inFIG. 28. As shown in curve I, the peak ratio increased with increasingpressure and reached a constant value 28 at 10000 kg/cm² pressure. InFIG. 28, curve J shows the peak ratio of a sample which was re-sinteredat 850° C. for 40 hours in air after pressing. Sintering is oftennecessary for using superconducting powders. From a comparison withcurve I, it is seen that, although by re-sintering the degree oforientation is slightly decreased, the pressure dependency was almostthe same.

Electrical resistance versus temperature curves for these samples weresimilar to those in FIG. 18. The resistance decreased with decreasingtemperature. At 110 K, a slight change of resistance was observed, andthe onset and zero resistance temperatures were 88 K and 83 K,respectively.

The Bi-Sr-Ca-Cu-O samples are known to have two super-conducting phasesof 110 K and 80 K. In this example, the sample showed a 80 K-classsuperconducting property.

EXAMPLE 23

Alkoxides, Bi(n-OC₄ H₉)₃, Sr(n-OC₄ H₉)₂, Ca(i-OC₃ H₇)₂, and Cu(i-OC₃H₇)₂, were mixed in a mole ratio of Bi:Sr:Ca:Cu=2:1.5:1.5:2 in 1-butanolas solvent. The solution was refluxed for 20 hours and pure water of 5to 10 times the total molar amount of Bi, Sr, Ca, Cu was titrated intoit with ultrasonic vibration. Then, the solution was refluxed again for10 hours to ensure complete hydrolysis. The oxide powder sample wasobtained by drying the solution.

The powder was pre-sintered at 800° C. for 10 hours, and ground intopowder again. The resultant powder was pressed into pellets at apressure of 4000 kg/cm² and examined by X-ray diffraction.

The peak ratio of 2Θ=23.2°/2Θ=27.5 was about 30. The degree oforientation was slightly higher than that in example 22.

POSSIBLE APPLICATIONS IN THE INDUSTRY

As described above, since fine oxide particles for superconductors aremade in a liquid phase at a temperature less than the boiling point ofthe solvent in the processes according to present invention, a fine andhomogeneous powder, which can be sintered at a low temperature, can beobtained.

By forming a buffer layer between a substrate and a superconducting filmto suppress the reaction of the super-conductor with the substrateduring the sintering process, materials which ordinarily cannot be used,such as silicon, silica glass or other ceramics, can be used as thesubstrate, and on the buffer-coated substrate, a c-axis oriented YBa₂Cu₃ O_(7-x) superconducting oxide film with good superconductingproperties can be fabricated.

Highly c-axis oriented superconducting film and bulk products can befabricated from the compounds with the starting compositions of Cu>50mole % and Ln<16.6 mole % in the Ln-Ae-Cu-O system.

Highly c-axis-oriented superconducting film and bulk products can befabricated from compounds with the starting compositions ofBi/(Bi+Ae+Cu)=5 to 40 mole %, Ae/(Bi+Ae+Cu) =15 to 70 mole %,Cu/(Bi+Ae+Cu)=24 to 64 mole %, and more preferably the startingcompositions of Bi/(Bi+Ae+Cu)=5 to 40 mole %, Ae/(Bi+Ae+Cu)=15 to 40mole %, Cu/(Bi+Ae+Cu)=24 to 64 mole %. C-axis oriented products areadvantageous for superconducting film and bulk in terms of high criticalcurrent density.

We claim:
 1. A process for preparing a superconducting oxide, comprising the steps of:a. separately dissolving in respective solvents alkoxides of a plurality of elements which constitute the superconducting oxide to provide respective alkoxide solutions; b. mixing the respective alkoxide solutions together to provide a mixed solution; c. subjecting the mixed solution to hydrolysis to provide a sol; d. evaporating the respective solvents from the sol to provide a concentrate; e. heating the concentrate at a temperature effective to provide an oxide powder; f. mixing the oxide powder with at least one of at least one additional oxide and at least one carbonate of elements which constitute the superconducting oxide to provide a mixed oxide powder; g. forming the mixed oxide powder into a continuous body; and h. sintering the continuous body.
 2. A process for preparing a superconducting oxide, comprising the steps of:a. separately dissolving in respective solvents alkoxides of a plurality of elements which constitute the superconducting oxide to provide respective alkoxide solutions; b. mixing the respective alkoxide solutions together to provide a mixed solution; c. subjecting the mixed solution to hydrolysis to provide a sol; d. evaporating the respective solvents from the sol to provide a concentrate; e. forming the concentrate into a continuous body; and f. sintering the continuous body; wherein said continuous body is a film, and wherein said film is formed on a buffer layer provided on a substrate.
 3. The process for preparing a superconducting oxide as claimed in claim 2, wherein the buffer layer is comprised of zirconia as at least the main constituent.
 4. A process for preparing an oxide superconductor having its crystalline c-axis oriented perpendicular to a surface thereof, the process comprising:a. separately dissolving in respective solvents alkoxides of at least one of yttrium and a lanthanoid, at least one alkaline earth metal, and copper to provide respective alkoxide solutions; b. mixing the respective alkoxide solution together to provide a mixed solution containing an amount of alkoxide for which Ln/(Ln+Ae+Cu)<16.6 mole %, Cu/(Ln+Ae+Cu)>50 mole %, and Ae/(Ln+Ae+Cu)=remaining mole %, wherein Ln is at least one of yttrium and a lanthanoid, and Ae is at least one alkaline earth metal; c. subjecting the mixed solution to hydrolysis to provide a sol; d. evaporating the respective solvent from the sol to provide a concentrate; e. forming the concentrate into a continuous body; and f. sintering the continuous body.
 5. The process for preparing an oxide superconductor as claimed in claim 4, wherein step e, forming, includes the further steps of evaporating the respective solvent from the concentrate to provide an oxide powder, sintering the oxide powder, grinding the oxide powder after sintering to provide a fine oxide powder, filling the fine oxide powder into a die, and pressing the fine oxide powder to provide the continuous body.
 6. The process for preparing an oxide superconductor as claimed in claim 4, wherein step e, forming, comprises applying the concentrate onto a surface of a substrate to form a film.
 7. The process for preparing an oxide superconductor as claimed in claim 4, wherein Ae is barium.
 8. The process for preparing an oxide superconductor as claimed in claim 4, wherein the oxide superconductor has a composition substantially equal to YBa₂ Cu₃ O_(7-x) in which x is a number indicating deviation from stoichiometry.
 9. A process for preparing a Bi-Ae-CuO series oxide superconductor having its crystalline c-axis oriented perpendicular to a surface thereof in which Ae is at least one alkaline earth metal, the process comprising:a. separately dissolving in respective solvents alkoxides of bismuth, at least one alkaline earth metal, and copper to provide respective alkoxide solutions; b. mixing the respective alkoxide solutions together to provide a mixed solution containing an amount of alkoxide for which mole % of Ae/(Bi+Ae+Cu) is less than the stoichiometric value of Ae of the oxide superconductor to be prepared; c. subjecting the mixed solution to hydrolysis to provide a sol; d. evaporating the respective solvents from the sol to provide a concentrate; e. forming the concentrate into a continuous body; and f. sintering the continuous body.
 10. The process for preparing an oxide superconductor as claimed in claim 9, wherein the mixed solution contains an amount of alkoxide for which Bi/(Bi+Ae+Cu)=5 to 40 mole %, Ae/(Bi+Ae+Cu)=15 to 40 mole %, and Cu/(Bi+Ae+Cu)=24 to 64 mole %.
 11. The process for preparing an oxide superconductor as claimed in claim 9, wherein step e, forming, includes the further steps of evaporating the respective solvents of the concentrate to provide an oxide powder, sintering the oxide powder, grinding the oxide powder after sintering to provide a fine oxide powder, filling the fine oxide powder into a die, and pressing the fine oxide powder to provide the continuous body.
 12. The process for preparing an oxide superconductor as claimed in claim 9, wherein step e, forming, comprises applying the concentrate onto a surface of a substrate to form a film.
 13. The process for preparing an oxide superconductor as claimed in claim 9, wherein the at least one alkaline earth metal is strontium and calcium.
 14. The process for preparing an oxide superconductor as claimed in claim 13, wherein the oxide superconductor has a composition substantially represented by Bi₂ (Sr,Ca)₃ Cu₂ O_(x) in which x is a number indicating deviation from stoichiometry. 